Field of the Invention
The present invention relates to electrophotographic image forming apparatuses such as copying machines, printers, etc.
Description of the Related Art
Fixing devices are used in electrophotographic image forming apparatuses such as copying machines, printers, etc., and there is known a fixing device in which a fixing roller is heated from an outer peripheral surface side. In general, such a fixing device includes a fixing roller, a heating rotary member configured to be in contact with the fixing roller to heat the fixing roller, and a pressing roller configured to be in contact with the fixing roller to form a nip portion. While being conveyed, a recording material on which a toner image is formed is heated at the nip portion to fix the toner image onto the recording material. Examples of the heating rotary member of the fixing device include a heating rotary member including a cylindrical film and a heater into contact with an inner surface of the film, a heating rotary member including a heating roller containing a halogen heater, etc.
Meanwhile, in the fixing device, a phenomenon called “offset” sometimes occurs in which a part of toner on a recording material is transferred to the outer peripheral surface of the fixing roller. Hereinafter, toner that has been offset will be referred to as offset toner. As the fixing roller is rotated, the offset toner may be transferred onto a surface of the heating rotary member and accumulated on the surface of the heating rotary member. The accumulated toner may form into a mass and occasionally return to the surface of the fixing roller to contaminate a toner image on the recording material.
To solve such a problem, Japanese Patent Application Laid-Open No. 2003-114583 discusses a fixing device in which the non-tackiness of a heating member (heating rotary member), i.e., an external heating member, with respect to toner on a recording material is set higher than the non-tackiness of a fixing roller. In the fixing device, the adhesive force between the offset toner and the fixing roller is stronger than the adhesive force between the offset toner and the heating member, so that the offset toner on the fixing roller does not adhere to the heating member and is likely to remain on the surface of the fixing roller. Thus, the offset toner on the surface of the fixing roller can be fixed onto the recording material and discharged as the fixing roller rotates.
However, it is not sometimes sufficient to give a mere difference between the non-tackiness of the external heating member and the non-tackiness of the fixing roller surface, and there still remains a problem that offset toner adheres to the external heating member.